In addition to widely-used amorphous silicon (a-Si), amorphous (non-crystalline) oxides comprising for example at least one kind of indium (In), gallium (Ga), zinc (Zn), tin (Sn) and so on are recently used for a semiconductor layer of a thin film transistor (TFT). Such an oxide semiconductor thin film comprising the oxide used for a semiconductor layer in a TFT has excellent semiconducting properties including high field-effect (carrier) mobility. In addition, the oxide semiconductor thin film has a wide optical band gap and film formability at low temperatures, which makes the formation on a plastic substrate or a film substrate possible.
However, as compared with a case of using a-Si, an oxide semiconductor thin film is liable to vary in terms of electrical characteristics and has a problem of low reliability when used for a display device such as a liquid crystal display or an organic EL display. It has been pointed out for example in Patent Document 1 that if there is a variation (shift) of threshold voltage (which is a gate voltage necessary to turn a transistor to the on-state) in a semiconductor element such as a transistor before and after light irradiation or bias-thermal stress test (BT test), the on/off state may not be controlled in a normal manner, resulting in significant deterioration of the reliability of the transistor. For an oxide semiconductor thin film, it is desired the threshold voltage (Vth) does not shift in I-V characteristics (drain current-gate voltage characteristics: variation in drain current with respect to change in gate voltage), that is excellent stress stability, when an oxide semiconductor thin film is subjected to the stress test in which a semiconductor element such as a transistor is subjected to being kept under light irradiation or continuous application of gate bias.
On the other hand, it is thus essential from the point of view to improving the productivity to evaluate properties of deposited oxide semiconductor thin films, to feedback the results of the evaluation, to develop a material for a semiconductor device or to adjust manufacturing conditions by optimizing a process, and to control film quality in the manufacturing process of the display devices. Desired for an oxide semiconductor thin film is a technology to accurately measure or predict and evaluate variation of the electrical property (amount of threshold voltage shift ΔVth) induced by the stress in a semiconductor device.
In a conventional evaluation method for evaluation of electrical properties of an oxide semiconductor thin film, the properties such as mobility and threshold voltage are measured in an actual TFT or a device simulating the TFT for which a gate insulator film or a passivation insulator film and contact electrodes are formed on the semiconductor thin film. It takes, however, enormous amount of time and cost to form contact electrodes in such contacting type evaluation methods that require actual fabrication of the devices. Forming the electrodes is also liable to induce additional defects in the oxide semiconductor thin film. In addition, it requires time to form the electrodes in the devices. It has thus been required from the point of view to improving fabrication yield to establish a contactless-type evaluation method in which formation of contact electrodes is not necessary.
In view of these circumstances, evaluation methods by microwave photo conductivity decay method (μ-PCD method) utilizing laser and microwave have been proposed as a method for evaluating the characteristics of a semiconductor thin film in a non-contact manner (Patent Document 2 and Patent Document 3).
Among these, the method according to Patent Document 2 has been proposed to evaluate the crystal quality of a crystalline semiconductor thin film such as polycrystal silicon. A specimen comprising the crystalline semiconductor thin film is irradiated with a laser. The crystal quality of the semiconductor thin film is evaluated by measuring the change in reflectance of the microwave which depends on the excess carriers generated by the laser irradiation.
Further, the technology of Patent Document 2 has been modified in Patent Document 3 so that irradiation conditions of excitation light is properly set for an oxide semiconductor thin film for the purpose of evaluating characteristics of the oxide semiconductor thin film which is amorphous. Specifically, Patent Document 3 discloses (I) a method comprising irradiating both excitation light and microwave radiation to a specimen on which an oxide semiconductor thin film is formed, measuring the maximum value (peak value) of reflected wave of the microwave radiation, which varies with the irradiation of the excitation light, from the oxide semiconductor thin film, then stopping the irradiation of the excitation light, and thereafter measuring a variation in the reflectance of the microwave radiation reflected by the oxide semiconductor thin film after the stopping, and evaluating mobility of the oxide semiconductor thin film by calculating lifetime value (1/e of change in the reflectance) from the measured value; and (II) a method of evaluating mobility of an oxide semiconductor thin film by irradiating both excitation light and microwave radiation to a specimen on which an oxide semiconductor thin film is formed, and measuring the maximum value (peak value) of reflected wave of the microwave radiation, which varies with the irradiation of the excitation light, from the oxide semiconductor thin film.